Staphylococcus aureus is a dangerous human pathogen responsible for both community acquired disease and nosocomial infections. S. aureus strains cause a variety of disease symptoms due to the production of a wide array of toxins. The emergence of strains resistant to multiple antibiotics, in combination with the horizontal transfer of toxin genes, poses a significant public health hazard. A large number of staphylococcal superantigens, including enterotoxins B, C and toxic shock toxin, have recently been shown to be encoded on a family of related, 15-20 kb pathogenicity islands (SaPIs). The SaPIs represent a novel type of mobile genetic element, which can be transferred at high frequency following helper phage infection. Studies of SaPIs, the prototype element, demonstrated phage-induced excision and replication of the pathogenicity island, followed by the packaging of the pathogenicity island into transducing virions. SaP1 transducing particles resemble those of the helper phage but have heads about 1/3 the volume of the plaque-forming helper phage particles, commensurate with the smaller size of the SaPH genome. SaPH mobilization is highly efficient (about 6 orders of magnitude higher than generalized transduction of chromosomal markers), and thus far has been demonstrated for a single helper phage, 80 alpha. A second superantigen pathogenicity island, SaPI2, is transduced by staphylococcal phage 80 at a frequency similar to that seen for SaPI1 with 80 alpha. The mechanisms underlying the phage-mediated excision, replication and encapsidation of these pathogenicity islands as well as the determinants of the observed phage specificity remain to be elucidated, and represent the long-term goal of this project. The proposed studies will establish the identity and origin of the virion proteins in the smaller SaPI transducing particles. Helper phage and SaPI genes involved in formation of the smaller SaPI capsids will be determined, and the role of a SaPI-encoded small terminase subunit in SaPI DNA packaging will be investigated. These studies will provide insight into the remarkable relationship between two accessory genetic elements that are responsible for the dissemination of staphylococcal enterotoxins. This knowledge will assist in the development of strategies to inhibit the transfer of toxin genes between Staphylococcus aureus strains and help slow the emergence of highly virulent, multiresistant organisms. [unreadable] [unreadable] [unreadable]